1. Field of the Invention
The present invention relates to a resin molding die for use in the molding-based resin encapsulation of a semiconductor device, and a production method for semiconductor devices employing such a resin molding die. In particular, the present invention relates to a production method for semiconductor devices employing a high-fluidity liquid thermosetting resin as an encapsulation resin, and a resin molding die for use with such a production method.
2. Description of the Related Art
Conventionally, resin encapsulation of semiconductor devices is achieved by: (1) casting molding, (2) transfer molding, or (3) injection molding.
(1) Casting molding. This technique involves: pouring a liquid thermosetting resin into a curing cast of PPS (polyphenylene sulfide), TPX (transparent plastic material for optics), or the like; mounting semiconductor devices and inserting wire-bonded lead frames; and heating the complex in an oven.
(2) Transfer molding. Under this technique, a resin called xe2x80x9cB stage resinxe2x80x9d, which results after the reaction of a thermosetting resin is interrupted halfway, is used. At molding, this resin is liquefied by the application of heat and pressure, poured into a heated casting die, and then allowed to thermally cure.
(3) Injection molding. This technique chiefly employs a thermoplastic resin as an encapsulation material. The encapsulation material is liquefied under a high pressure, poured into a heated casting die, and then allowed to cool.
Conventionally, the injection molding technique has only employed thermoplastic resins. However, the development of liquid thermosetting resins which have an faster reaction rate has allowed an injection molding technique employing a liquid thermosetting resin.
However, the aforementioned conventional techniques have the following problems.
(1) When the casting molding technique is used to mold an epoxy resin (a chief material which is currently used as an encapsulation resin), there is a problem in that the epoxy resin requires a long curing time for a polyaddition reaction to occur; this step bottlenecks the production efficiency. Recently, radical reaction type resins which require a relatively short curing time are also studied. However, since this type of resins exhibit a substantial sink due to cure-related shrinkage, such resins have not been put to practical use.
(2) Since the transfer molding technique uses a B stage resin, i.e., a resin which results after the reaction of a thermosetting resin is interrupted halfway, such a resin must be stored in a frozen state, adding to the transportation and storage costs. Such a resin also requires a long curing time for a polyaddition reaction to occur.
(3) When the injection molding technique is used for encapsulating a semiconductor device with a thermoplastic resin, the high resin injection pressure used may break the wires which are used for wire-bonding a semiconductor device and a lead frame. In order to prevent this problem, a method has been proposed (Japanese Publication for Opposition No. 4-40870) in which a low-pressure injection is first performed to produce resin for protecting the wires, and then the actual molding of an external shape follows. However, the proposed technique requires some consideration in connection with a molding cycle time, and an increased number of molding dies must be used; therefore, this technique has yet to be introduced to actual mass production lines. Moreover, the high level of heat resistance and humidity resistance which is required for a semiconductor device encapsulation resin cannot be expected from a transparent thermoplastic resin due to its resin characteristics.
On the other hand, when the injection molding technique is used for encapsulating a semiconductor device with a liquid thermosetting resin, it is possible to protect wires by previously allowing a resin to adhere to the wires and curing the resin (called a xe2x80x9cpre-dip curing techniquexe2x80x9d). However, according to this technique, the viscosity of the resin has been lowered due to the die heat when the resin is injected at a high injection pressure. As a result, the resin may leak from the parting faces of the molding die, allowing flashes to form.
A resin molding die according to the present invention includes: a cavity: a resin inlet through which a liquid resin to be cured is injected into the cavity; and an air vent through which air is released to an exterior space of the resin molding die during injection of the resin, the air vent being provided on an opposite side from the resin inlet with respect to the cavity.
In one embodiment of the invention, the resin is a thermosetting resin; and the air vent has an interspace of such dimensions that when the resin molding die is heated to or above a curing temperature of the thermosetting resin, air is allowed to be released to the exterior space of the resin molding die but that the resin cures and remains within the resin molding die.
In another embodiment of the invention, the air vent has an interspace of such dimensions that the resin flows at a pressure which is in the neighborhood of a pressure imposed by the weight of the resin.
In still another embodiment of the invention, the air vent has an interspace of such dimensions that the resin being filled in the cavity expels air to the air vent, and that the resin cures within the air vent without leaking into the exterior space of the resin molding die.
In still another embodiment of the invention, the resin is a thermosetting resin; and at least a portion of the resin inlet is in an open state when the resin molding die is heated to or above a curing temperature of the thermosetting resin during injection of the resin.
A method for producing a semiconductor device according to the present invention includes the steps of: setting a semiconductor device in a cavity of a resin molding die; injecting a thermosetting resin from a resin injection nozzle into the cavity, through a resin inlet of the resin molding die and a gate, at a pressure which is equal to or greater than a pressure imposed by the weight of the thermosetting resin and which is equal to or less than about 2 kg/cm2; and curing the thermosetting resin to encapsulate the semiconductor device within the thermosetting resin.
In one embodiment of the invention, the step of injecting the thermosetting resin includes lowering a viscosity of the thermosetting resin to about 3000 cps or less so that the thermosetting resin can pass through the gate at a pressure which is equal to or greater than the pressure imposed by the weight of the thermosetting resin and which is equal to or less than about 2 kg/cm2.
In another embodiment of the invention, the step of injecting the thermosetting resin includes injecting the thermosetting resin while heating the resin molding die to or above a curing temperature of the thermosetting resin.
In still another embodiment of the invention, the step of injecting the thermosetting resin includes injecting the thermosetting resin while maintaining the resin molding die below a curing temperature of the thermosetting resin: and the step of curing the thermosetting resin includes heating the resin molding die to or above the curing temperature of the thermosetting resin.
In still another embodiment of the invention, the step of injecting the thermosetting resin includes providing a space between the resin inlet and the resin injection nozzle so as to ensure that the resin inlet is not closed off by the resin injection nozzle when the resin molding die is heated to or above a curing temperature of the thermosetting resin during injection of the resin.
In still another embodiment of the invention, the step of injecting the thermosetting resin includes cooling the resin injection nozzle so as to be maintained at a temperature sufficiently below a curing temperature of the thermosetting resin.
In still another embodiment of the invention, the thermosetting resin is a resin which is obtained by adding a radical polymerization initiator or a cationic polymerization initiator to at least one main component selected from the group consisting of: a liquid epoxy resin; a liquid vinyl ester resin; an allyl resin; and a low-viscosity unsaturated polyester resin.
In still another embodiment of the invention, the thermosetting resin is a transparent resin; and the semiconductor device is a light receiving/emitting device.
Epoxy resins, which have conventionally been used as thermosetting resins for semiconductor device encapsulation, require a long curing time for a polyaddition reaction to occur. On the contrary, the present invention allows the use of a radical reaction-type resin or a cationic polymerization-type resin, so that the reaction time can be substantially reduced. Such resins, which will not react unless a certain level of heat is applied thereto, do not need to be stored in a frozen state unlike the B-stage resins.
Such resins, when used for casting molding, may exhibit some sinking due to cure-related shrinkage. However, when such resins are used for injection molding, the continuous supply of resin prevents the occurrence of sinking because any lack of resin is always compensated for. Thus, injection molding is suitable for the molding of a rapid-curing resin. Yet, using a liquid thermosetting resin for the encapsulation of a semiconductor device still invites problems such as flash formation due to a high injection pressure.
Thus, the inventors conducted vigorous research to develop a technique for injecting a liquid thermosetting resin into a molding die at a low injection pressure, thereby arriving at the present invention.
Thus, the invention described herein makes possible the advantages of (1) providing a resin molding die which enables low-cost resin molding without allowing resin sink, wire breakage, or flash formation to occur; and (2) providing a production method for semiconductor devices using such a resin molding die.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.